Plate heat exchangers are frequently used as evaporators for evaporation of refrigerants circulated in refrigeration systems. Normally, such a refrigeration system comprises a compressor, a condenser, an expansion valve and an evaporator, all of which are coupled in series. In a plate heat exchanger which, is used as an evaporator in a system of this kind the plates are often brazed or welded together. However, gaskets may also be used as a sealing means between adjacent heat transfer plates.
A problem which arises in connection with a refrigeration system of the above referenced type, is that refrigerant entering the inlet channel of the plate heat exchanger is not evenly distributed to the different evaporation flow paths in the interspaces between the heat transfer plates. One reason for this may be that the refrigerant, after having passed through the expansion valve, is already partly evaporated when it enters the inlet channel, and does not remain in the state of a homogenous liquid/vapour mixture during the passage along the whole of the inlet channel, but tends to partly separate into streams of liquid and vapour, respectively.
Uneven distribution of refrigerant to the different evaporation flow paths in the plate heat exchanger results in ineffective use of parts of the plate heat exchanger. Moreover, the refrigerant may become unnecessarily overheated. Furthermore, some channels may be flooded by liquid refrigerant and there is also a risk that some liquid may be present at the outlet.
In order to avoid the problem of uneven distribution of the refrigerant in a plate heat exchanger of the above mentioned type it has previously been suggested in SE 8702608-4 to arrange a restriction means in each passage between the inlet channel of the plate heat exchanger and each plate interspace forming an evaporation flow path for the refrigerant. The restriction means could be a ring or a washer provided with a hole and being arranged between adjacent pairs of the heat transfer plates around the port hole. Alternatively, the restriction means could be a pipe provided with multiple holes or apertures and being arranged in the inlet channel of the plate heat exchanger. As a further alternative it has also been suggested in SE 8702608-4 to create restriction means as an integral part of the heat transfer plates by folding the plate edge portions delimiting the inlet ports of two adjacent heat transfer plates to abutment against each other, edge to edge. In a small area however, inlet openings are formed allowing refrigerant to pass into the flow paths between adjacent plates.
Plate heat exchangers provided with restriction means of the above mentioned kind give rise to several difficulties during the manufacture thereof. The use of separate rings or washers has resulted in problems with the location of the rings or washers in the correct positions when a plate heat exchanger is assembled. A restrictions means in the form of a pipe has the disadvantage that it must have a length adapted to the number of heat transfer plates included in the plate heat exchanger and it must also be correctly positioned in relation to the inlet passages leading into the flow paths between the heat transfer plates. Folding of port edge portions of the plates has also been shown to be unpractical, depending on the fact that it is difficult to obtain well defined inlet openings leading into the plate interspaces as proposed in SE 8702608-4.
Another solution to the problems encountered in connection with uneven distribution of refrigerant to the different evaporation flow paths in the plate heat exchanger, is to provide a well defined inlet passage for restriction of the incoming medium. Plate heat exchangers with such restriction means are disclosed in WO 95/00810 and WO 97/15797.
In the plate heat exchangers according to WO 95/00810 and WO 97/15797, the inlet and outlet channels along the plate package forms ducts with walls having successive peaks and valleys. This particular shape of the channel along the plate package, however, has a disadvantageous impact on the flow of the fluids forcing the fluid to contract and expand, resulting in turbulence and backflows, influencing the quantity and quality of the refrigerant mixture entering the flow paths between adjacent plates and causing pressure drop. Specifically, this is very critical for the refrigerant inlet channel along the plate package, as it negatively influences the distribution of the refrigerant along the plate package.
Ideally, the distribution of refrigerant along the plate package should assure equal mass flow rate with the same vapour quality of refrigerant in each and every refrigerant channel between the heat transfer plates. However, in reality it is quite difficult to achieve such performance since the physical and flow dynamic conditions of the fluid change as the fluid proceeds along the plate package.